Pulverized coal has been successfully burned in suspension in furnaces by tangential firing methods for a long time. The technique involves introducing the coal and air into a furnace from the four corners thereof so that it is directed tangent to an imaginary circle in the center of the furnace. This type of firing has many advantages, among them being good mixing of the fuel and air, stable flame conditions, and long residence time of the combustion gases in the furnace. In recent times, it has become important to minimize air pollution as much as possible. Thus, some proposed changes have been made to the standard tangential firing method. One such arrangement is set forth in our pending patent application Ser. No. 786,437, now abandoned, entitled "A Control System and Method for Operating a Tangentially Fired Pulverized Coal Furnace", filed on Oct. 11, 1985. That application proposes introducing pulverized coal and air tangentially into the furnace from a number of lower burner levels in one direction, and introducing coal and air tangentially into the furnace from a number of upper burner levels in the opposite direction. By this arrangement, better mixing of the fuel and air is accomplished, thus permitting the use of less excess air than with a normal tangentially fired furnace, which generally is fired with 20-30% excess air. The reduction in excess air helps minimize the formation of NO.sub.x which is a major air pollutant of coal-fired furnaces. It also results in increased efficiency of the unit. Although the above firing technique reduces NO.sub.x, it does have some disadvantages. Since the reverse rotation of the gases in the furnace cancel each other out, the gases flow in a more or less straight line through the upper portion of the furnace, increasing the possibility of unburned carbon particles leaving the furnace due to reduced upper furnace turbulence and mixing. In addition, slag and unburned carbon deposits on the furnace walls can occur. These wall deposits reduce the efficiency of heat transfer to the water-cooled tubes lining the walls, increases the need for soot blowing, and reduces the life span of the tubes.